I've talked my company (small one, some 10 employees) into using TWiki as a collaboration environment. It is used to document projects, store documents in a structured way, etc.

It's even been setup in such a way that project status can be exported to HTML and taken to a client on a laptop. While security is possible and present, one need not use it which lowers the threshold to use the system: it's just fun to use. The version control system is an excellent feature as well.

My co-workers are quite happy with it and it gets used a lot as opposed to previous attempts to set up such an environment.

Just my 2 cents, -gjb-